1. Field of the Invention
The invention relates to a method for selecting at least one transponder or sensor in RFID or remote sensor systems having a plurality of transponders or sensors, in particular systems with a plurality of reading devices.
2. Description of the Background Art
Automatic identification methods, also known as Auto-ID, have become widespread in recent years in many service fields, in procurement and distribution logistics, in trade, in production, and in material flow systems. The goal of Auto-ID in this context is the comprehensive provision of information concerning persons, animals, objects and goods.
One example of such Auto-ID systems is the now widely used chip card, in which a silicon memory chip is powered, read, and if necessary reprogrammed, by a reading device by mechanical galvanic contact. In this context, the acquisition device is generally referred to as a reading device regardless of whether it can only read data or can also write it.
In RFID systems, power can be supplied to the data carrier—the transponder—not only by galvanic contact, but also in a non-contact manner using electromagnetic fields in the radio frequency range (RF, radio frequency).
RFID systems include two basic components, namely the transponder or—in the case of a remote sensor system—the sensor, i.e. an integrated circuit (IC) with a coupling element such as a dipole antenna as a transmitting and receiving means, and the reading device (also called the base station), which is typically a high-frequency module (transmitter/receiver) and likewise has a coupling element. The reading device regularly supplies the transponder or the sensor, which need not have its own voltage supply, with energy; data are transmitted both from the reading device to the transponder (forward link) and in the opposite direction (return link). Such RFID systems, whose range is significantly greater than 1 m, use electromagnetic waves in the UHF and microwave regions. These systems mostly use a backscatter method, named for its physical principle of operation; in this method, a part of the energy arriving at the transponder from the reading device is reflected (scattered back, hence backscattering) and may be modulated in the process in order to transmit data: The IC receives, through the coupling element, a high frequency carrier, some of which it transmits back to the reading device through suitable modulating and backscattering devices.
When multiple transponders or sensors—hereinafter referred to by the generalized term, tags—are located in an RF field of a reading device, the reading device must carry out an appropriate selection process prior to data transmission if the data transmission is only to take place between one tag or one group of tags and the base station. In particular, the reading device must single out the tags, for example in order to determine their identification. These processes are also known as “anticollision methods.”
To begin with, stochastic, ALOHA-based anticollision methods are known in this regard (see Finkenzeller, “RFID Handbuch,” Hanser Verlag, third edition, p. 210 ff), which has been published in English by John Wiley & Sons., however these have inherent speed disadvantages.
In addition, each of the tags in the simplest case is distinguished by an identifying bit sequence, known as a unique ID or UID, that has been statically predefined by the manufacturer and is stored in the tag. Using this UID, a reading device can address the tags individually or as a group. A suitable method is known from U.S. Pat. No. 5,856,788, for example. In this method, selection takes place on the basis of a bitwise comparison of the unique, statically defined identifying bit sequence with a selection bit sequence transmitted by the reading device. Such selection methods are also referred to as deterministic, in contrast to the aforementioned stochastic methods.
Another deterministic anticollision routine is known from DE 102 04 346, which corresponds to U.S. Pat. No. 7,102,488, and which is incorporated herein by reference.
Nonetheless, it can easily happen in open systems that a unique UID is no longer guaranteed on account of the variety of existing UID specifications. For this reason, a German application DE 103 36 308, which corresponds to U.S. Publication No. 2005024186, and which is incorporated herein by reference, expands the identifying bit sequence of the tags by the addition of a random element, so that a deterministic selection can take place even when the identifying bit sequence is not unique.
In general, however, for any type of deterministic selection method, it must be considered a disadvantage that the reading device has to regularly transmit control signals in the form of clock ticks or notches (modulation dips) at the bit boundaries for control purposes.
In the backscattering-based RFID and remote sensor systems the reading device in typical applications transmits an RF carrier at 30 dBm and a modulated signal of appropriate bandwidth whose spectrum is produced by the carrier and the modulation (through the sidebands produced by the clock tick transmission). The tags radiate back only a portion of the transmitted or received energy, so that the reading device receives a signal at approximately −70 dBm, which is only slightly above the ambient noise for a noise spectrum of other RF services. The boundary levels for GSM or comparable applications are at −36 dBm. Sidebands from the reading device produced in RFID applications must be below −54 dBm, which is to say they are sometimes stronger than the desired signal, but they can easily be filtered out in the frequency domain. Accordingly, the received channel of the reading device must be designed to be extremely sensitive; however, it must be considered a disadvantage here that every reading device thus of necessity also receives the interfering, generally asynchronous transmissions from other reading devices in the system together with their associated sidebands.
Not least on account of the aforementioned reasons, the processing speed is generally one of the most important optimization goals in RFID applications. This is especially true in the area of UHF (ultra high frequency) and microwave systems, where field gaps can regularly occur as a result of reflections. Moreover, especially in long-range systems, distant and/or moving objects, for example in package distribution or stacker applications, produce continuous background noise and result in further problems on account of differing reflection characteristics.
Consequently, it is known in conventional RFID systems, such as described in ISO standard 18000-6, for example, to use commands that are kept as short as possible for communicating with tags by a reading device, with such commands generally being used to transmit only default settings such as a (memory) address on the tag. Thus, for example, it is customary to address the ID of the tag with such commands. Deviation of the application from such settings, e.g. to query data other than the tag ID, has the result that either the reading device must transmit long command sequences (separation by means of an anticollision routine followed by a READ command to read out the data in question) or an anticollision command is issued containing appropriate parameters, e.g. the bit address to be read out. If, in addition, the aforementioned ambient conditions also change during the course of the anticollision procedure, further time-consuming adjustments, such as the data stream coding, must also be made.
Such a conventional procedure thus results in prolonged anticollision routines with the aforementioned adverse effects on data transmission in RFID systems.
In WO 2004/021257 A, a method for data transmission is described, which functions asynchronously, at least in part.